@fladd
Consider to use a bit more fresh ISOs linked in the [Stable Update] 2021-10-16 - Haskell, Python, KDE Gear, KDE Framework, Thunderbird - #3 by philm
It is official.
It is a rare case, but to be sure after download consider to check hash sums of your downloaded image(s) to be 100% sure that the ISO file(s) came undamaged.
Also, directly after you load your installed copy consider to get a fresh servers mirrors to to get updates from:
sudo pacman-mirrors -B stable -a -c Sweden -P https
or use any other country checked in the stable column of the https://repo.manjaro.org/
May be that time you will be more lucky 